To observe the expression of CD40/CD40L on peripheral blood mononuclear cells (PBMC) in patients with condyloma acuminatum (CA), flow cytometry was employed to examine the expression of CD40 and CD40L on PMBC in 36 patients with CA and 20 healthy controls. Our results showed that mean level of CD40 expression in CA patients was significantly lower than that in the controls (6.58% +/- 2.74% vs 14.81% +/- 6.12%, t = 5.703, P < 0.05); the average level of CD40L in CA patients was also significantly lower than that in the controls (0.73% +/- 0.54% vs 2.67% +/- 2.43%, t = 3.532, P < 0.05). Our results suggest that the reduced costimulatory interaction of CD40 and CD40L in CA patients may be one of the important factors responsible for the low cellular immunity.